Portable sheet metal bending brakes are utilized in the bending of relatively thin gauge sheet metal material, such as aluminum siding, and must be portable, yet rugged for transport. The brakes usually are constructed of an array of "C"-shaped main castings held in assembly by an appropriate frame, the castings being provided with a clamping and bending structure by which the sheet metal is clamped and bent at the open ends of the "C" castings. Various structures have been proposed to clamp the workpiece in position, typically by a transverse slide mechanism as shown in the prior art, e.g., Rowan et al U.S. Pat. No. 3,147,791 or Marsh U.S. Pat. No. 3,161,223. Where the bending of thick workpieces is attempted, the "C" castings deflect and open up, so that the workpiece is not firmly held in position for bending. As a result, the utility of such prior art brakes has been severely limited in terms of the thickness of material which can be bent.
Various other types of clamping mechanisms have been proposed for increasing the gauge of the metal which can be bent. For example, Barnack U.S. Pat. No. 3,481,174 proposes an over-center clamping mechanism, but the close tolerances necessary to an adequate over-center clamp has made this structure difficult to manufacture and maintain under actual operating conditions in the field. Another attempt at a different clamping structure is shown in Break U.S. Pat. No. 4,081,986, where a cam locking structure is provided. This cam-actuated brake utilized a segmented shaft arrangement with a single clamping arm adjacent a single casting with the cam, in effect, cantilevered from the casting on a small pivot pin. This proposed brake suffers from various structural deficiencies and has never been commercially acceptable. Also, difficulties are encountered in maintaining a constant clamping load along the entire brake length.
A need exists in the art for a rugged, portable sheet metal bending mechanism which can bend heavier gauge material, which can bend such material at angles in excess of 90.degree. to 105.degree., and which can make complex, hemmed, or deep section bends of the type desirable in present construction techniques.